1. Field of the Invention
The present invention relates to emission control for combustion processes, and more particularly, to reducing the gaseous sulfur-containing compounds and particulate matter which result from firing pulverized coal in a combustion device.
2. Description of the Prior Art
The use of sulfur-bearing coals containing mineral matter in various combustion installations (steam boilers, furnaces, and the like) results in the production of sulfur-containing compounds, such as sulfur dioxide, sulfur trioxide, and fly ash particulates. These are pollutants and must be removed to a certain extent from the exhaust gases prior to release into the atmosphere. Various methods are known to attempt to deal with this problem.
The sulfur-containing compounds are usually absorbed by different sorbents injected either into the firebox or into a scrubber installed downstream of the combustion device. This causes increased mineral matter in the flue gas discharged by the system, and thus requires additional use of energy (heat and electrical power) for the particulate removal process.
The fly ash particulates are removed from the exhaust gases by various means, such as filtration of the flue gas, or use of an electrostatic precipitator. The efficiency of an electrostatic precipitator depends on many factors, including the electrical conductivity of the particulate matter. Increased conductivity results in improved precipitation performance. The electrical conductivity in turn depends in part on the sulfur concentration in the particulate matter. In general, conductivity increases with increasing sulfur concentration in the particulate matter. Because of this, in order to improve the electrostatic precipitator performance when low sulfur coal is burned, sulfur is often introduced with the coal, or sulfur trioxide is injected into the flue gas.
The problem of achieving a reduction in sulfur-containing compounds and an increase in efficiency of particulate removal, while eliminating the use of additives and additional scrubbing processes or gas particulate cleaning processes, has not been adequately resolved.
The concept of cofiring natural gas with coal to improve emission performance is not new. Compared to coal, natural gas contains negligible sulfur and no mineral matter. It is known that partial replacement of coal with natural gas will result in a reduction in sulfur compounds and particulates proportional to the reduction in sulfur and mineral matter input. It is also known that cofiring of natural gas and oil yields a plume which is almost invisible.
The most significant development work on cofiring technology was conducted as a result of the Gas Research Institute's Cheswick Station project. It was discovered that cofiring natural gas with coal could result in a percentage reduction in sulfur dioxide emissions which was greater than the percentage reduction in sulfur and mineral matter input. Further, if a small amount of gas is fired simultaneously with coal, carbon burnout is improved, resulting in improved electrostatic precipitator performance and reduced plume opacity.